An Experiment Study On Reducing Cadmium And Lead Uptake From Polluted Soil By Rice And Wheat With Biochar Amendment

Cd and Pb are two of heavy metals that is a great threat to human health with migrating in the soil-plant-food chain, and farmland Cd and Pb pollution has become the outstanding problem of environmental health. In South China, the rice production areas of heavy metal contaminated become serious with most of the acid soil, which is posing potential public health risk for human and food safety. So, it becomes hot research that heavy metal pollution effective repair measures in paddy rice for safe production. Restoration measures include engineering repair, of physical chemistry repair, phytoremediation and so on. Though the effects engineering repair were complete, stable, etc., the defects of it were complex process, high cost and decreased soil fertility etc.; Physical chemistry repair were quickly and could produce food with repair processing, but some amendments maybe bring secondary pollution, and exist risk of activation; Phytoremediation were low cost, clean remove contaminants, but the biomass of plant was small and needed a long repair time. So it was necessary to find a cost-effective methods and technologies, in order to reduce the heavy metal uptake by plants and improve food security.Biochar produced by that the biomass (Such as:wood, crop residues, nut shell, coconut shell, bark, wastepaper etc.) is heated to moderate temperatures, between about 350 and 500℃(giving the process the name "low-temperature pyrolysis"), under complete or partial exclusion of oxygen. It is one of the low density low-temperature pyrolysis material and a new soil amendment. There are such characteristics:high stability, corrosion resistance, and greater ability retain fertilizer compared with other forms of organic fertilizer. What is more, the biochar could increase the soil pH to hold the soil nutrition elements and heavy metals because of the large surface area and surface negative electric charge. So the biochar could be used to mitigate climate change, improve the soil environment, relieve the environmental pollution and have the potential for pollution control. In order to explore the biochar adsorption characters and stabilization mechanism to the heavy metals, we design lab experiments and field experiments to analyze adsorption and desorption capacity of the biochar and the effect immobility the heavy metals to reduce the crop uptake. We hope that it can provide a scientific basis for biochar industry and farmland pollution environmental control technology. The main results just as follows:1. The lab experiments include the biochar adsorption and desorption under Cd concentration sequence, time sequence, initial pH sequence, different partial sizes and dosage. The biochar adsorption efficiency reached over 98% in Cd solution, and adsorption amount unit increased with the Cd concentration increase in the biochar adsorption isotherm under different Cd concentration. It is inverse proportion that the biochar adsorption efficiency and adsorption amount, namely the adsorption amount increased and the adsorption efficiency decreased, but adsorption amount increased rate faster than adsorption efficiency decreased rate. The biochar adsorption rate was fast and reached equilibrium in 10 min. The biochar removal efficiency increased with pH increasing for Cd, and great effect on the Cd solution after adsorption reached equilibrium and the solution pH increased by 10.4%-73.9% compared to initial pH.The smaller partial size, the larger adsorption ability of the biochar. When the partial size was smaller than 0.84 mm, the increase rate decreased of the adsorption amount. When the biochar partial size was larger than 0.84 mm, the biochar adsorption amount unit decreased by 21.8% sharply. The adsorption amount unit decreased and adsorption rate increased with biochar amount increasing. When the biochar concentration was at 8 g L-1, adsorption rate reached over 90%.The biochar desorption rate was between 1.83% and 5.39%, with the biochar amount increasing, desorption amount unit and desorption rate decreasing. It was indicated that the Cd is very strong to combined biochar, not easy to desorption. So it was more likely the specific adsorption of the biochar. The biochar adsorbed much more Cd than Pb in contaminated soil with the biochar application.2. A field experiment was set to test the effect of biochar on passivating soil Cd/Pb and reducing crop Cd/Pb uptake. The amounts of 0,10,20,40 tone biochar per hectare were supplied in the seriously polluted paddy soil before rice planting in 2009. The results of analyzing the soil and plant samples in 2009 and 2010 were that, biochar application improved the soil pH by 0.16-0.38 units.After the biochar application in rice season, the CaCl2 extraction soil available Cd were decreased by 32.5%-52.5% (2009) and 21.6%-48.6%(2010), but no significant difference of soil available Pb in two years. The DTPA extraction soil available Cd and Pb were decreased by 21.6%-48.6% and 11.9%-20.3% only in 2010. Compared to control treatment, the available Cd/Pb contents in wheat season soil were decreased by 23.4%-40.1% (CaCl2 extraction Cd), 11.1%-12.0%(DTPA extraction Cd) and 15.0%-18.8% (DTPA extraction Pb) under biochar applications.Although there was no significant effect of biochar on crop yield, rice and wheat grain Cd contents was significantly decreased by 16.8-45.0% (rice grain in 2009),39.9%-61.9% (rice grain in 2010) and 24.8%-44.2% (wheat grain in 2010), and the grain Pb contents were not changed. Meanwhile, different rice/wheat tissues Cd contents were significantly decreased under biochar application. Compared the Cd uptake in grain per hectare among different treatments, Cd grain uptake per hectare were significantly decreased by 45.8%-64.5% (rice grain in 2009),39.8%-64.5% (rice grain in 2010) and 32.1%-42.3% (wheat grain in 2010) under biochar application. And the effects of biochar are better than calcium-magnesia phosphate fertilizer experiment in the aside field.Under biochar application, Cd accumulation coefficients were significantly decreased by 39.8%-66.1% (rice) and 29.0%-49.6%(wheat), and Cd transfer coefficients were also significantly decreased by 15.9%-38.5% (rice) and 5.9%-36.6% (wheat). It indicated that inhibiting the Cd absorption from root and transfer to grain were both key ways to reduce the grain Cd contents with biochar application.The study above showed that the passivating of biochar on available Cd in contaminated soil was significant, and the effect of biochar on soil pH and soil available Cd can last at least two years.3. The experiment was carried out on moderately polluted cropland to study the feasibility of biochar application on the safe food production of agriculture.The four biochar supply grads (0,10,20,40 t ha-1) were set to probe the feasibility of biochar application on the safe food production of agriculture. In moderately polluted field, the pH was 5.36, and the DTPA and CaCl2 extraction Cd were 0.55 mg kg-1 and 2.72 mg kg-1, respectively; the brown rice Cd content was up to 0.72 mg kg-1. After the application of biochar, the soil pH were increased by 0.65-1.08 units, and the CaCl2 extraction Cd/Pb were both decreased by 48.0%-70.9% and 17.0%-34.0%, respectively; but there were no significant difference in DTPA extraction Cd/Pb among the treatments. The rice yield was not significantly changed with biochar application, but the brown rice Cd/Pb contents were decreased by 18.8%-43.3% and 13.7%-38.1%, respectively. Moreover, the plant tissues Cd/Pb contents of rice were also significantly decreased. The rice grain Cd content was decreased to 0.41 mg kg-1, which was close to the limit value of CCFAC (Codex Committee on Food Additives and Contaminants). Meanwhile, both of Cd and Pb accumulation coefficients were reduced in different periods; The Cd accumulation coefficients was significantly 66.2%-78.3% in Ripen stage, and 41.0%-76.8% in Filling stage and Jointing stage; The Pb accumulation coefficients was between 17.4% and 64.1% in whole growth stage. The Cd transfer coefficients between 23.9% and 47.3% in whole growth stage with biochar application.The results of field experiments on seriously polluted and moderately polluted croplands showed that, the application of biochar reduced the contents of soil available Cd significantly, and the available Pb contents were also reduced in moderately polluted cropland duo to its low pH. The effects of biochar on yield were not significantly, however, the Cd contents in grain and plant tissue in two experimental fields and the Pb contents in the crops in moderately polluted field were all reduced under biochar application. Through analyzing the accumulation coefficient and transfer coefficient, it was drawn that the biochar can inhibit the transfer of heavy metals from root to grain. Moreover, the Cd/Pb contents of crop grain planted on moderately polluted field did not exceed the limit value of CCFAC.4. There was significant correlation between the soil available Cd/Pb concentrations and pH values with the application of biochar in contaminated soils, which indicated that the increasing of soil pH may be the most important factor on heavy metals availability. Furthermore, long effort of biochar on the immobilization of the heavy metals in contaminated soils was found from the results of available Cd/Pb and Cd/Pb content grain in two consecutive years, and the Cd/Pb uptake and transfer process was then decreased.To sum up, both in the seriously and moderately polluted cropland, the application of biochar can significantly reduce the availability of heavy metals in the soil and the absorption in crops; moreover, the two years experimental results showed that the effect of biochar stabilized Cd/Pb at least last two years. The mechanisms of stabilization soil heavy metal and crop uptake reduction with biochar application were that the biochar has huge surface area which is benefit for absorbing and immobilizing numerous heavy metals in soil rapidly, and also its strong alkaline can increase soil pH to reduce the chemical mobility of heavy metals. Thereby, the biochar effects inhibited the migration of heavy metals in soil-plant system and reduced rice grain Cd/Pb uptake greatly, so as to improve the food security.